Electronic identity tags using transponders are used in the art to identify and monitor objects remotely. One particular utility for such transponders is to identify livestock, which can wear transponders uniquely identifying each of the individual animals. Fixed interrogating devices can then emit an electromagnetic wave transmission to interrogate the transponders when the animal is in range of the interrogator, to automatically identify animals as they pass certain stations. Such identification transponders can be commonly employed as a means of providing herd management, security systems, and inventory or process control and to monitor the functions of the animal such as feeding, body temperature, and milk production. Such electronic transponder tags commonly transmit fixed information that merely identifies the object or animal. Such transponders are also having identification in other uses in which it is desired to transmit automatically certain other information about the object or individual to which the transponder has been associated.
In the prior art it has generally been the case that the transponders which have been developed can generate a code which uniquely identifies a particular animal. For example, in U.S. Pat. No. 4,463,353 to Kuzara, such an electronic tag identification system is disclosed which is passive, receiving its power from a source of interrogating electromagnetic RF radiation, and which sends back a coded identifying signal in response to the interrogating pulse. U.S. Pat. No. 4,114,151 to Raymond et al. discloses a similar passive transponder system in which the passive transponder is powered by the RF signal received from the interrogating device, and in which the transponder is capable of generating a code to the interrogating device so as to digitally identify the transponder to the interrogating device. Many other examples in the art have known of the general concept of passive transponders capable of sending a digital code to the interrogating unit. Such devices are normally single state devices operable only in a single mode and accordingly must be hard wired or fixed in their programming to transmit a single data pattern which is normally not alterable by the user. Accordingly, a desirable feature in such transponder is the ability to incorporate the transmission of more and varied information as the management of the animals becomes more sophisticated requiring the desirability of additional monitoring of other parameters, such as temperature, time in lactation, weight and other factors related to the day-to-day life of the animal.